This is a double-blind, placebo-controlled, multi-center, crossover study to investigate the efficacy and safety of the glucagon receptor antagonist BAY 27-9955 in the treatments of patients with type 2 diabetes mellitus inadequately controlled with insulin therapy. The primary hypothesis to be tested is that BAY 27-9955 treatment will reduce mean fasting plasma glucose by 20 mg/dl or more, or reduce the mean plasma glucose during the four hours after the morning test meal by 18 mg/dl. BAY 27-9955 pharmacokinetic data will also be obtained. The overall-design of the study is as follows; a two-week screening period, a 21-day treatment period, a 14-day wash out period, and then a cross over to the alternative treatment for 21 days. Approximately seven patients will be enrolled at six active sites, until a total of 41 patients have entered the study